I don not have a Ruger Old Army but I've done some research into the subject & both Kirst & R&D work as they say they do in that it'll convert your cap & ball revolver to fire .45 Colt metalic cartridges that is loaded to cowboy pressures.

The Cylinders are rated to & waranteed to light cowboy loads or under 800 fps., you can load these with Trail Boss or Black Powder "which is popular" to the very closely reflect what the orriginal loading was & still be under the rated pressures.